As early as 1651 William Harvey, the court physician of Charles I of England, showed that all life came from the egg. It was much later, however, that the part played by the sperm and egg cell in fertilization was carefully worked out. It is to Harvey, too, that we owe the beginnings of our knowledge of the circulation of the blood. He showed that blood moved through tubes in the body and that the heart pumped it. He might be called the father of modern physiology as well as the father of embryology. A long list of names might be added to that of Harvey to show how gradually our knowledge of the working of the human body has been added to. At the present time we are far from knowing all the functions of the various parts of the human engine, as is shown by the number of investigators in physiology at the present time. Present-day problems have much to do with the care of the human mechanism and with its surroundings. The solution of these problems will come from applying the sciences of hygiene, preventive medicine, and sanitation. In the preceding chapters of this book we have learned something about our bodies and their care. We have found that man is able within limitations to control his environment so as to make it better to live in. All of the scientific facts that have been of use to man in the control of disease have been found out by men who have devoted their lives in the hope that their experiments and their sacrifices of time, energy, and sometimes life itself might make for the betterment of the human race. Such men were Harvey, Jenner, Lister, Koch, and Pasteur. [Illustration: Edward Jenner, the discoverer of vaccination.]